This invention relates to a process of producing a semiconductor thermally sensitive switching element utilizing a pnpn junction.
Semiconductor thermally sensitive switching elements to which the present invention is concerned are disclosed, for example, in Japanese laid-open patent application No. 146,283/1975 and utilize the pnpn junction responsive to a temperature in excess of a predetermined magnitude to cause their OFF state to disappear thereby to be shifted to their ON state. It is well known that thyristors with the pnpn junction are widely utilized as switching element but such thyristors are permitted to be employed only within a temperature range wherein a rise in junction temperature does not cause a decrease in off-state voltage or wherein any thermal cause does not affect the turn-on. That temperature at which this rise in temperature causes the disappearance of the OFF state is far higher than a rated maximum junction temperature. Thus there have not been previously available thyristors enabled to effect the switchover at low temperatures, for example, at the temperature ranging from room temperature to 100.degree. C. or lower by means of the thermal action.
In order to provide semiconductor thermally sensivitve switching elements effecting the switchover at such sufficiently low temperatures and low in power loss, it is required to increase the reverse leakage current through the pn junction that is reversely biased in their OFF state as described in the cited application. This can be done by disposing a low lifetime region of carriers in a portion of this pn junction. The formation of that low lifetime region has been already accomplished either by introducing atoms of a heavy metal such as gold (Au) into the main face portion of planar structures adjacent to the surface of the abovementioned pn junction disposed therein according to diffusion or ion implantation technique to form recombination centers in the diffused or implantated portion or by irradiating semiconductor substrates with radioactive radiation having high energy such as X rays, .gamma.-rays, electron beams or the like to form lattice defects therein to increase the number of recombination centers. However, it has been difficult to apply either of these measures to the practical manufacturing of semiconductor thermally sensitive switching elements for the following reasons: Heavy metals, for example, gold forming recombination centers in semiconductors have generally the diffusion coefficient very high in both semiconductors and oxide films. Further suitable diffusion masks have not been available and so on. As a result, it has been practically difficult to introduce atoms of such a heavy metal into only near to the surface region or one portion of the pn junction by diffusion technique with good reproducibility. Also the introduction of gold atoms or the like through the use of ion implantation technique has not necessarily provided effective means because, in order that the introduced gold atoms are operated as recombination centers through their activation, heat treatment at an elevated temperature is required after the ion implantation. During this heat treatment the gold atoms would have been diffused into a region other than the desired portion of the pn junction for the reasons similar to those above described.
Also radioactive radiations have generally the very strong penetration power in both semiconductor crystals and surface passivations films such as a silicon dioxide film. As a result, any suitable mask for radioactive radiation has not been yet provided. Accordingly, it has been difficult to form a low lifetime region only in the desired region of semiconductor substrates adjacent to the surface of the desired pn junction or one portion thereof through the irradiation with radioactive radiation. In addition, it has been very difficult to form the low lifetime region only in one specified region of semiconductor substrates according to any of known techniques. It is very desirable to enable the formation of a low lifetime region in a predetermined portion of the particular semiconductor substrate alone.
Accordingly, it is an object of the present invention to provide a new and improved process of producing a semiconductor thermally sensitive switching element effecting the switchover at a sufficiently low temperature by forming a semiconductor region in which the lifetime of carriers is short in one portion of a region of a desired pn junction disposed in a semiconductor substrate.